Wireless power is becoming increasingly popular to transfer energy to a device, for example, to charge a battery in the device and to do so without having to plug the device in to a power source. Power is transferred through the inductive coupling of a magnetic field generated by a transmitter and delivered wirelessly to a receiver (e.g., in a battery-operated device). A pair of coils of wire (one in the transmitter and the other in the receiver) may be used to wirelessly transfer the energy from transmitter to receiver. Charging pads are available which employ this technology. In a charging pad, a battery-operated device such as a smart phone is placed on the charging pad and can be charged without making an electrical connection to the phone.
A concern with wireless power transfer systems is the heating of metallic objects which may be unintentionally exposed to the magnetic field. For example, a coin, candy wrapper or car keys inadvertently might be placed on the charging pad along with the smart phone to be charged. Such objects, because they are metal, may absorb the energy being wirelessly transmitted and intended for the phone. As a result, the metal object warms up. The worst possible place for such an object is between the phone and the charging pad. A metal object in that location may be heated to temperatures that can melt the plastic surfaces of the phone and the charging pad and pose a risk of fire.